1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor (hereinafter also referred to as a “photoreceptor”), which is used for image formation in the electrophotographic system and can be exposed to short-wavelength light, and to an image formation device provided with the photoreceptor.
2. Description of the Related Art
An electrophotographic system image formation device (hereinafter also referred to as an “electrophotographic device”) using electrophotographic technologies to form an image bears the responsibility of a part of high-speed information processing system devices and has made significant progress in recent years. Among these system devices, the electrophotographic system using light as the recording probe has been significantly improved in the qualities of print outputs and in reliability along with improvement in the qualities of a light source itself. Accordingly, these technologies promote not only evolution of usual printer outputs but also evolution of copy machines, so that the importance of these technologies are increased, and therefore, a growing and successive demand for these technologies are expected also in the future.
The developments of high quality printers and copy machines including those giving high-quality color images are currently awaited with full anticipation. Then, examples of the trend of technologies in the attainment of this purpose include technologies for “light sources (exposure light beam) more reduced in diameter and formations of a more highly precise latent image and a more highly developed image” and technologies for “stabilization of a photoreceptor capable of coping with the above anticipation”.
To attain reduction in the size of an exposure light beam, which is the former requirement, the use of shorter wavelengths is effective. In the case of using a short-wavelength laser (LED) having, for example, a center oscillation wavelength which is nearly one-half that of the near-infrared laser (LD) as the writing light source, the spot diameter of the laser beam on the photoreceptor can be considerably reduced in theory as shown by the following equation.d∝(π/4)(λf/D)  (A)                wherein d is a spot diameter on the photoreceptor, λ is a wavelength of laser light, f is a focal distance of a fθ lens and D is a diameter of the lens.        
Therefore, reduction in the size of the exposure beam is very advantaged in working to improve a writing density of a latent image and a resolution.
However, use of such a short wavelength LD poses some problems in stable operation of a photoreceptor because the wavelength of the short-wavelength LD is shorter than the center oscillation wavelength of a conventional exposure member.
A first problem is concerned with stability of the photoreceptor. Specifically, since a current writing wavelength is longer than 450 nm, writing is allowed from the surface side of the photoreceptor even if a charge transport material of the charge transport layer is yellow. However, in the case of the short-wavelength LD when the charge transport material blocks the writing light, not only photosensitivity is deteriorated, but also deterioration of the charge transport material itself is promoted, with the result that the very thing of the function of the photoreceptor is deteriorated.
Therefore, when the short-wavelength LD is used, it is essential to use an optically near-colorless one as the charge transport material and it is necessary to develop such a charge transport material. To deal with this problem, a material is selected which has a molecular structure providing a nearly colorless transparent film after the film is formed, from among the charge transport materials which have been developed so far.
A second problem is concerned with stability of the charge generation material. Specifically, as compared with a light absorption and charge generation process along with light absorption for the near-infrared laser, those for the short-wavelength light are largely different in a point of interaction with a material. In other words, energy of the short-wavelength light is clearly larger than that of the current infrared laser when the same charge generation material is used, which allows the occurrence of such an idea that excess energy causes a secondary action during the course extending to the process of creating carriers by light. Though the details of the process are not clear, this is given as a problem concerning stability of the photoreceptor.
Methods resulting from the grappling with an improvement in resolution by using a short-wavelength laser are seen in examples including a method described in Japanese Unexamined Patent Publication No. HET 9(1997)-240051 and methods using a combination of various charge generation materials as shown in, for example, Japanese Unexamined Patent Publication No. 2000-47408 and Japanese Unexamined Patent Publication No. 2000-105479. However, neither “the achievement of high resolution” nor “securance of stability required for the photoreceptor” have been realized yet.
Almost all of the market of a group of materials used for a photoreceptor are currently occupied by organic materials in point of performances and/or costs. However, inorganic compounds featured by, for example, durability and/or stability may also be used as the photoreceptor to be introduced into a special market.
Therefore, hybridization capable of drawing features of the both is desired earnestly. As to inorganic compounds that have been already put into practical use, various inorganic compounds are introduced for a performance of a photoreceptor.
Further, there is an example in which metal oxide particles are introduced into a charge generation layer to intend to achieve an improvement in stability as described in Japanese Unexamined Patent Publication No. HEI 5(1993)-249708.
Moreover, there are descriptions concerning the influence of the light absorption of a metal oxide in the undercoat layer on the characteristics of sensitivity of the photoreceptor in Japanese Unexamined Patent Publication No. 2007-233347. However, it may be said that its effect is still insufficient.